Infared Schwank Gas Heater "Exploding"

[ivandykeman]

100,000 BTU/h Schwank Propane Gas Infared Heater, suspended from ceiling. 11"W.C gas pressure.
I set the t-stat to make the circuit everything runs as it should up until about 5 minutes into operation. Seems like the gas builds up literally explodes and repeats this sequence until the flame sensor cools off enough to shut the gas valve. Unit will retry 4 times before it locks out.

Gas pressures are as they should be
Ceramic tiles are properly installed, sealed, cleaned recently
Ignition module has been replaced(dosent matter because unit lights properly)
gas valve has been replaced once before.
installed in a relatively clean atmosphere.

the whole building has the exact same heaters and only 2 have thsi problem..i believe i have covered every avenue and can't seem to diagnose the problem. I am pointing towards a manufacturers defect in sizing???

[slimwoodie]

hhhmmmnnn ...
i don' think so ...

[ivandykeman]

yeah i didnt think so myself but iv tried fooling with the combustion air sleeve(operates the same way with or without one installed). theres no draft in this area strong enough to blow the flame out so i think i've ruled out everything i would know at my knowlege level

[bigjohn79]

dose this happen every time or is it intermittent.

I am just the local Idiot around here but it sounds like a venting problem of some kind.

are these units new?? did they ever work right??

[Moose]

LP units - have you checked the piping - size/arrangement? When it 'blows' what is the man. press.? - Mains pressure?

[ivandykeman]

the problem occurs everytime there is a call for heat..goes into about 5 mins of normal operation then boom...lights again..boom lights again and so on and so on until the flame sensor cools down enough to shut the gas valve

gas pressures are as they should be..(as per spec sheet) the units dont vent anywhere because they are infared..they have a pourous ceramic tile arrangement where the gas seeps through and ignites..the tiles turn a bright orange and produce radiant heat.

talking to lennox about a manufacturer design prob at the moment...any input is greatly appreciated.

[kuhnhunter]

Do they have proper clearances, I had some do this before and it was because they were mounted too close to the ceiling, also make sure that you don't have some sort of draft blowing on this unit, I would think if it was a design problem you would have problems at your initial ignition.

[rev]

I usually just send them things back to manufcturer for repairs. Not scwank but Ive had the same problems with them. They usually say that they peplaced the refractory. So im guessing its the heat gasket between the ceramic and metal gets eroded. I ask them why and they say make sure the angle of them is right. seems usually between 30 and 45 deg. but I think it can also be caused from being them getting to hot. as said before being too close to ceiling without enough airflow.

[hvacplumb]

11" wc you are using propane i assume

[ivandykeman]

thanks for the input...from tlaking to lennox today i believe that they are twisted in such a manner that they did recieve damage to the refractory gasket causeing the air/propane mixture to ignite prematurely ..and yes it is 11 " w/c propane gas..any other suggestions are welcome

[D'Laine]

It's been years since I worked on these units. Little s.steel rods to hold the ceramic tiles in place? Gaskets are important.
If you can watch the gas pressure at the time the popping occurs, without blowing your self off the ladder? Maybe it fluctuates, when other heaters cycle on or a big demand from some other appliance.

Tiles have the right size of holes for that model? Maybe someone put in the wrong ones.
Cracks in the manifolds? At the 5 minute mark?, well it would take that long to get good and hot and a hairline crack can open right up.

Just what's going on inside that thing at 5 minutes?

D.

[ivandykeman]

thanks for the input everyone..have narrowed the problem down to a cracked weld in the manifold that has been causeing all the grief..once i replace it i will post the results for information.